Morphology and Dynamics of Dense Nanometric Precursor Films of Polymer Melts

Abstract

Nanometer-thick supported films of polymer melts spontaneously form and spread around sessile droplets that are deposited on oxidized silicon wafers. At steady state, the films become dense and adopt a uniform thickness, which is equal to twice the gyration radius of the free polymer. Remarkably, this law applies to a wide variety of melts and does not depend on the polymer chemistry nor on the surface state (oxide layer thickness, temperature, presence of water adsorbed, etc.). We show that existing theoretical descriptions cannot reproduce this experimental result. Conversely, the evolution toward this equilibrium state witnesses the specificity of the interactions at stake in these confined polymer films. The chain spreading dynamics can be modeled by taking into account both the polymer/surface friction and the polymer/polymer friction.

Complexity in Wetting Dynamics

“…48 But the disjoining pressure itself does not decide the existence of a prewetting film. In this framework, consistent with experimental data, a positive disjoining pressure sets the contact angle of a drop at equilibrium on the film to zero (total wetting 6,7,49 ) while a conjoining pressure sets it to a nonzero value (pseudopartial wetting 38,39,43 ).…”

“…Experimental work has further shown that prewetting films consist of 2D gases at their leading edge 41 connected to the droplet by dense thicker films appearing at later times. 39 These results clarify the existence of a precursor film as soon as the spreading parameter is positive, that is, when the adsorption of molecules from the liquid on the bare solid is energetically favored. Within the film, independently of the naked substrate energy, molecular interactions that produce either a disjoining or a conjoining pressure between the two film interfaces have an effect on the shape of the film/air interface from the film tip to the droplet, including its macroscopic contact angle.…”

“…These observations demonstrate that the thermally activated motion of molecules at the contact line is very effective at pulling a film of molecular thickness out of the drop (Figure ) rather than setting the whole liquid corner into motion. Experimental work has further shown that prewetting films consist of 2D gases at their leading edge connected to the droplet by dense thicker films appearing at later times . These results clarify the existence of a precursor film as soon as the spreading parameter is positive, that is, when the adsorption of molecules from the liquid on the bare solid is energetically favored.…”

“…From Liu et al Depending on the end groups, both the equilibrium contact angle and friction ζ vary. (c, d) Ellipsometric microscopy images from Schune et al showing a prewetting precursor film of nanometer thickness h spreading (c) around a small droplet of PI–OH over time and (d) at the edge of a large droplet of PI-CH3. The equilibrium contact angle, the dynamics, and the precursor film thickness (or molecular surface density) all strongly depend on the terminal groups.…”

“…An example of spreading dynamics of a droplet coexisting with a precursor film is shown in Figure . For droplets consisting of polarizable polymer melts spreading on silicon wafers, observations of the wetting dynamics at the macroscopic scale prove the existence of an equilibrium contact angle while measurements at the microscale reveal the presence of a nanometer-thick precursor film. , These systems are examples of the so-called pseudopartial wetting state , that can be detected only if observations at the nanoscale are performed. The analysis of both the dynamics and the statics should then integrate this peculiar boundary condition.…”